Observatories Team Up to Reveal Rare Double Asteroid

New observations by three
of the world's largest radio telescopes have revealed that an asteroid discovered last year is actually two
objects, each about 3,000 feet (900 meters) in size, orbiting each other.

Three of the world's largest radio telescopes team up to show a rare double asteroid. 2017 YE5 is only the fourth binary near-Earth asteroid ever observed in which the two bodies are roughly the same size, and not touching. This video shows radar images of the pair gathered by Goldstone Solar System Radar, Arecibo Observatory and Green Bank Observatory.

Near-Earth asteroid 2017
YE5 was discovered with observations provided by the Cadi Ayyad University Morocco Oukaimeden Sky Survey on Dec. 21, 2017, but no details about the asteroid's physical
properties were known until the end of June. This is only the fourth
"equal mass" binary near-Earth asteroid ever detected, consisting of
two objects nearly identical in size, orbiting each other. The new observations
provide the most detailed images ever obtained of this type of binary asteroid.

On June 21, the asteroid 2017 YE5
made its closest approach to Earth for at least the next 170 years, coming to
within 3.7 million miles (6 million kilometers) of Earth, or about 16 times the
distance between Earth and the Moon. On June 21 and 22, observations by NASA's
Goldstone Solar System Radar (GSSR) in California showed the first signs that
2017 YE5 could be a binary system. The observations revealed two distinct
lobes, but the asteroid's orientation was such that scientists could not see if
the two bodies were separate or joined. Eventually, the two objects rotated to
expose a distinct gap between them.

Bi-static radar images of the binary asteroid 2017 YE5 from the Arecibo Observatory and the Green Bank Observatory on June 25. The observations show that the asteroid consists of two separate objects in orbit around each other.
Credit: Arecibo/GBO/NSF/NASA/JPL-Caltech Full image and caption

Scientists at the Arecibo Observatory in
Puerto Rico had already planned to observe 2017 YE5, and they were alerted by
their colleagues at Goldstone of the asteroid's unique properties. On June 24,
the scientists teamed up with researchers at the Green Bank Observatory (GBO)
in West Virginia and used the two observatories together in a bi-static radar
configuration (in which Arecibo transmits the radar signal and Green Bank
receives the return signal). Together, they were able to confirm that 2017 YE5
consists of two separated objects. By June 26, both Goldstone and Arecibo had
independently confirmed the asteroid's binary nature.

The new observations
obtained between June 21 and 26 indicate that the two objects revolve around
each other once every 20 to 24 hours. This was confirmed with visible-light
observations of brightness variations by Brian Warner at the Center for Solar System Studies in Rancho
Cucamonga, California.

Radar imaging shows that the two objects are
larger than their combined optical brightness originally suggested, indicating
that the two rocks do not reflect as much sunlight as a typical rocky asteroid.
2017 YE5 is likely as dark as charcoal. The Goldstone images taken on June 21
also show a striking difference in the radar reflectivity of the two objects, a
phenomenon not seen previously among more than 50 other binary asteroid systems
studied by radar since 2000. (However, the majority of those binary asteroids
consist of one large object and a much smaller satellite.) The reflectivity
differences also appear in the Arecibo images and hint that the two objects may
have different densities, compositions near their surfaces, or different
surface roughnesses.

Scientists estimate that among near-Earth
asteroids larger than 650 feet (200 meters) in size, about 15 percent are
binaries with one larger object and a much smaller satellite. Equal-mass
binaries like 2017 YE5 are much rarer. Contact binaries, in which two similarly
sized objects are in contact, are thought to make up another 15 percent of
near-Earth asteroids larger than 650 feet (200 meters) in size.

Radar images of the binary asteroid 2017 YE5 from NASA's Goldstone Solar System Radar (GSSR). The observations, conducted on June 23, 2018, show two lobes, but do not yet show two separate objects.
Credit: GSSR/NASA/JPL-Caltech
Full image and caption

The discovery of the binary nature of 2017 YE5
provides scientists with an important opportunity to improve understanding of
different types of binaries and to study the formation mechanisms between
binaries and contact binaries, which may be related. Analysis of the combined
radar and optical observations may allow scientists to estimate the densities
of the 2017 YE5 objects, which will improve understanding of their composition
and internal structure, and of how they formed.

Study contributors

The Goldstone observations were led by Marina
Brozovic, a radar scientist at NASA's Jet Propulsion Laboratory in Pasadena,
California.

Anne Virkki, Flaviane Venditti and Sean
Marshall of the Arecibo Observatory and the University of Central Florida led
the observations using the Arecibo Observatory.

Patrick Taylor of the Universities Space
Research Association (USRA), scientist at the Lunar and Planetary Institute,
led the bi-static radar observations with GBO, home of the Green Bank Telescope
(GBT), the world's largest fully steerable radio telescope.

The Arecibo, Goldstone and USRA
planetary radar projects are funded through NASA's
Near-Earth Object Observations Program within the Planetary Defense Coordination Office (PDCO), which manages the Agency's Planetary Defense Program. The
Arecibo Observatory is a facility of the National Science Foundation operated
under cooperative agreement by the University of Central Florida, Yang
Enterprises, and Universidad Metropolitana. GBO is a facility of the National
Science Foundation, operated under a cooperative agreement by Associated
Universities, Inc.

In addition to the resources NASA puts into
understanding asteroids, the PDCO also partners with other U.S. government
agencies, university-based astronomers, and space science institutes across the
country, often with grants, interagency transfers and other contracts from
NASA. They also collaborate with international space agencies and institutions that
are working to track and better understand these smaller objects of the Solar
System. In addition, NASA values the work of numerous highly skilled amateur
astronomers, whose accurate observational data helps improve asteroid orbits
after discovery.

More information about
asteroids and near-Earth objects is at these sites: